The present invention relates to a cathode ray tube such as a color picture tube or a color display tube including a cathode having an electron emissive material layer, and particularly to a cathode ray tube having improved high-current operational characteristics and reduced a warm-up time required for formation of an image after a heater has been turned on.
A cathode ray tube such as a color cathode ray tube used for a monitor at a terminal of office automation equipment, for example, generally has a vacuum envelope comprised of a panel, a neck and a funnel for connecting the panel and the neck, a phosphor screen comprised of three-color phosphor picture elements coated on an inner surface of the panel, and an electron gun housed in the neck.
The electron gun for the cathode ray tube has three cathodes for generating the three electron beams in a horizontal direction and a plurality of electrodes located downstream of the three cathodes and spaced in the direction of travel of the electron beams for forming a main lens. The three electron beams from the cathodes enter the main lens, are accelerated and focused appropriately, and then impinge upon the phosphor screen.
The phosphor screen comprises three-color phosphor picture elements fabricated in the form of dots or stripes and arranged at a predetermined pitch, and a color selection electrode such as a shadow mask is closely spaced from the phosphor screen between the phosphor screen and the electron gun.
In this type of cathode ray tubes, each cathode in the electron gun is provided with an electron-emissive material layer coated on a base metal and a heater for heating the base metal such that electrons are emitted from the electron-emissive material layer.
Some electron-emissive material layers employ a multilayer structure suitable for high-current operation and for prevention of peeling of the electron-emissive material layers off the base metal, a two-layer structure, for example.
In the two-layer structure, a first layer on the base metal side comprises alkaline earth metal oxide powders converted from triple carbonates containing Ba, Sr, and Ca carbonates, ((Ba,Sr,Ca)CO3), and a second layer, i.e., an upper layer, comprises the same alkaline earth metal oxide powders as in the first layer and 1 to 3 weight percent of a rare earth metal oxide dispersed in the alkaline earth metal oxide powders. A barium scandate Ba2Sc2O5, BaSc2O4 or Ba3Sc4O9, a composite oxide of Ba and Sc, is used as the rare earth metal oxide dispersed in the second layer.
The operating temperature for the electron-emissive material layer comprised of these alkaline earth metal oxides (BaO, SrO, CaO) and the rare earth metal oxides dispersed therein is usually 1000 K.
A reducing agent contained in the cathode base metal diffuses to the surface of the cathode base metal at this temperature, and reduces the alkaline earth metal oxide BaO. The thicker the base metal, the longer the reducing agent continues to diffuse to the surface of the base metal, resulting in increased cathode life, as disclosed in detail in Japanese Patent Application Laid-open No. Hei 5-12983 (laid-open on Jan. 22, 1993).
A known cathode base metal is made of material containing Ni as a main component and incorporated with a low concentration of a reducing element such as silicon (Si) or magnesium (Mg).
Properties of the base metal are related to the mechanism of electron emission from a cathode, and there are various opinions on the mechanism of the electron emission.
Generally it is thought that the reducing agent in the base metal reduces the barium oxide to produce free barium, and this free barium diffuses in the electron-emissive material layer, forms a donor level in the alkaline metal oxide and thereby emits electrons.
Usually, the emission life is determined by the exhaustion of the reducing agent in the cathode base metal and the evaporation of the electron-emissive material BaO. As for the exhaustion of the reducing agent in the cathode base metal, the thicker the cathode base metal, the longer the reducing agent continues to diffuse to the surface of the cathode base metal, resulting in longer cathode life.
Because of the above, a cathode base metal of 0.19 mm in thickness has been popular, following a specification for cathodes of types previous to cathodes of the type having the rare earth metal dispersed in the electron-emissive layer.
The evaporation of the electron-emissive material BaO is determined by the temperature of the electron-emissive material layer, but the exhaustion of the reducing agent in the cathode base metal is reduced by the effects of the barium scandate dispersed in the electron-emissive layer.
A high concentration of free barium in the electron-emissive material layer suppresses reduction of barium oxide by the reducing agent in the base metal, and thereby reduces the exhaustion of the reducing agent.
In the above-explained prior art, sufficient consideration has been given to emission life characteristics by employing the two-layer electron-emissive material layer and dispersing the rare earth metal oxide in the electron-emissive material layers but no consideration has been given to a warm-up time required for formation of an image on a cathode ray tube such as a color display tube after an image display set such as a monitor has been switched on. This warm-up time will be hereinafter referred to as the image-forming warm-up time.
The image-forming warm-up time is a time required for the electron-emissive material layer to reach a required temperature and is determined by the heat capacity of the heater-cathode system.
Especially, in the case of a color display tube used for a monitor of information equipment such as a personal computer (Pc), there is a tendency for heaters to be automatically turned off during a waiting time when the information equipment is not used, for the purpose of power saving, and consequently, the image-forming warm-up time causes a problem when the information equipment is used again after the waiting time.
Empirically, it is desirable that a time required for the screen brightness to reach 50% of the required brightness is limited to eight seconds or less (or a time required for the phosphor screen to become faintly luminous must be limited to three to four seconds) after power turn on, and if the time exceeds eight seconds, the operator sometimes feels irritated.
Power saving is also essential in view of energy saving and the protection of environment, and therefore there is a demand for reduction of the image-forming warm-up time after heater power turn on following the waiting time.
The present invention solves the above problem, and it is an object of the present invention to provide a cathode ray tube such as a color display tube capable of retaining the basic characteristics such as high-current operation and long emission life and reducing the image-forming warm-up time.
To accomplish the above object, in accordance with an embodiment of the present invention, there is provided a cathode ray tube comprising a vacuum envelope including a panel portion, a neck portion and a funnel portion for connecting the panel portion and the neck portion, a phosphor screen formed on an inner surface of the panel portion, and an electron gun housed in the neck portion and including a cathode having an electron-emissive material layer formed on a surface of a cathode base metal, the electron-emissive material layer comprising: a first layer made of an alkaline earth metal oxide on the surface of the cathode base metal, a second layer being an alkaline earth metal oxide layer containing at least one rare earth metal oxide in a range of 0.1 to 10 weight percent, the at least one rare earth metal oxide having a particle size distribution in which the number of particles having a maximum diameter over 5 xcexcm is one or none, the number of particles having a maximum diameter in a range of from 1 xcexcm to 5 xcexcm is in a range of from 2 to 30, as measured in an area of 45 xcexcmxc3x9745 xcexcm at a center of a top surface of the second layer, the maximum diameter Dmax being defined as a perpendicular projection, onto a horizontal direction, of tangents to extremities of a profile of each of the particles, the second layer being formed on a surface of the first layer; the cathode base metal being made chiefly of nickel and containing at least one reducing agent, and a thickness of a portion of the cathode base metal in contact with the electron-emissive material layer being in a range of 0.10 to 0.16 mm.